Single-unit studies are done in the vestibular system of the squirrel monkey (Saimiri sciurens). The aim is to describe the information encoded in peripheral afferents and its modification by central (including efferent) pathways. The long-range goal is to determine the functional contributions made by the individual components of the system to its overall operation. We are also interested in the mechanisms of sensory transduction. Four projects are proposed: (1) We shall compare the response of peripheral afferents to natural stimulation and to galvanic currents. The study should elucidate the transduction mechanisms responsible for the afferents' discharge characteristics. (2) The response of peripheral neuron to rotations about an earth-horizontal axis will be investigated since the corresponding reactions of the entire system are not understood at the level of the peripheral receptors. (3) Studies of the functional organization of the vestibular nuclei will continue, with emphasis placed on central otolith pathways and the possibility of canal-otolith interactions. (4) Recordings will be done in the alert, behaving monkey in order to investigate the discharge characteristics of efferent vestibular neurons and efferent-induced modifications of afferent activity.